The present invention relates to a microminiature, force-sensitive switch having a mechanical switching element formed in a silicon wafer.
The important electronic properties of silicon in integrated circuit technology are well established. More recently, the mechanical properties of silicon wafer components have been investigated. The inventor herein has previosly described the construction of an electrostatically deflectab1e silicon-beam device for use as an electrical switch or modulating element. Petersen, K. E., "Micromechanical Membrane Switches on Silicon", IBM J. Res. Develop., Vol. 23 No. 4, pp. 376-385 (1979). The deflectable beam device described includes a thin (about 0.35 micron) silicon dioxide membrane coated with a conductive metal layer. The device functions as an active circuit element, in the sense that the switching function requires that a deflection voltage be applied. The device thus operates as an electrostatic version of an electromagnetic relay. Applications of silicon wafer mechanical devices to ink-jet nozzles and charge plates, a capillary gas chromatograph system, a miniature biomedical accelerometer, an optical bench for positioning fiber-optic components and lasers, and a microminiature Joule-Thompson cryogenic refrigerator have been described and are referred to in the above-cited paper.
Micromechanical devices formed in silicon wafers provide a number of advantages over other types of micromechanical elements, which are typically formed of metal membranes. Silicon membrane elements can be batch fabricated silicon intergrated circuit technology, and, as such, can be made to high accuracy and high reliability at relatively low cost. The fabrication techniques are readily adaptable to different design requirements. Silicon membranes also appear to be relatively fatigue-resistant. Initial studies on the operating behavior of micromechanical silicon elements, reported in the above-cited paper, indicate that continued flexing of the single crystal silicon elements is less likely to result in fatigue and breakage than in metal membranes.
The present invention addresses the need, in microminiature sensing systems, for a reliable, low-cost switch device capable of sensing, within a desired range, an external condition such as pressure, acceleration or temperature. The switch includes a silicon wafer having a reduced-thickness, deflectable membrane which is responsive to force-related changes in the external condition being measured. Movement of the membrane from a relaxed condition to more strained conditions establishes electrical contact between a common terminal and first one and then progressively more switch-state terminals in the switch, providing a digital measurement of the external condition acting on the switch, according to the number of switch states which are "closed" and "open". The switch may also function to monitor a selected threshold level of pressure, acceleration or temperature. The threshold level selected may be any one of several levels corresponding to one of the several switch states in the switch. Since the switch is a passive electrical element, wherein the switch state is determined solely by its response to an external condition, the switch can be used in a passive sensing system, such as a miniature accelerometer implanted within a body or a pressure sensing system located within a vehicle tire.
In a preferred switch construction, the silicon wafer is composed of a boron-doped silicon layer formed on a silicon substrate and having a silicon epitaxial layer formed over the boron-doped silicon layer, and the deflectable member is composed of the boron-doped silicon layer and a reduced-thickness portion of the epitaxial layer. The deflectable member is preferably between about 10 and 100 microns thick, and of between about 0.5 and 2 mm in length.
In one general embodiment, the deflection membrane takes the form of a diaphragm formed in a central region of a silicon wafer and adapted to bulge outwardly, from a central diaphragm region, in response to an increasing force-related external condition. The diaphragm-membrane switch may be constructed to include a fluid-tight chamber covering one side of the diaphragm, allowing the switch to respond to pressure differentials across the diaphragm. The diaphragm switch may also be used for measuring acceleration, by attaching a mass to the diaphragm, or for measuring temperature, by including in the diaphragm a metal layer having a substantially different thermal coefficient of expansion than that of the diaphragm's silicon layers.
In another general embodiment, the deflectable member takes the form of an elongated beam which may be either anchored at its opposite ends, for deflection from a central beam region, or at one end only, in cantilevel fashion, for deflection from its free end region. The beam-configuration switch may be used for digitizing or for monitoring a threshold temperature or acceleration level.
The invention further contemplates a passive, microminiature diaphragm switch having a reduced-thickness deflectable diaphragm, a stationary contact member, a pair of switch terminals, and electrical contacts associated with the confronting surfaces of the diaphragm and contact member for connecting the two switch terminals conductively when the diaphragm is moved from a relaxed to a preselected bulged condition, in response to a change in an external condition such as temperature, pressure or acceleration.
It is a general object of the present invention to provide a microminiature switch for use in digitizing or monitoring a threshold level of an external condition such as pressure, temperature or acceleration.
Another object of the invention is to provide such a switch which may be fabricated, using silicon-wafer fabrication techniques, to have a maximum switch response in a selected pressure, acceleration or temperature range.
Yet another object of the invention is to provide such a switch which operates as a passive circuit element.
A more specific object of the invention is to provide, for use in determining or detecting different-external pressure states, a passive microminiature switch containing a silicon diaphragm formed in a silicon wafer.
These and other objects and features of the present invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.